Ubi's site about Rome, the Eternal City

By the end of the Republican times, the conflict between plebeians and patricians led to the stiff confrontation of two parties: the democratic one, led by Caius Marius, and the aristocratic one, at the head of which was Cornelius Silla. Marius allowed census-free citizens to enter the army, substantially making it (from a citizen army) a mercenary army, conquering the approval of the population.
Silla was supported by the Senate: he won Marius and proclaimed himself dictator for the life (notice that the term did not necessairily have the modern, negative connotation). This was a first, almost mortal stab at the democracy (by the current law, the title of "dictator" could be assumed only in case of war and for time span of six months).

To prevent the outburst of a civil war, some years after the voluntary resignment of the office by Silla himself, the power was given to a triumvirate (in latin, trium = three, vir = man): the triumvirs were Gnaeus Pompeius Magnus (Pompeius, in brief, a man form the army), Marcus Licinius Crassus (usually simply indicated as "Crassus", a rich man chosen to represent his class), Gaius Julius Caesar (head of the democratic party).
Nevertheless, because of the rivalry between Caesar and Pompeius, peace did not last for long; the political struggle turned into the dreaded civil war and Pompeius was defeated.

Crassus died in the war against the Parthians and Caesar made the Senate to give him a special status by which his person became sacred and inviolable (tribunitia potestas), the office of pontifex maximus (the supreme religious appointment) and, finally, the lifetime dictatorship with full military and civil powers.

Caesar diminished the power of the Senate to almost nothing, taking Rome a step away from being an empire, an absolute, military and religious monarchy.
A plot of senators -60, according to Plutarch-, probably the best known in history, in the Ides of March of 44 a.C., led to his death and prevented the emanation of many reforms he had in mind.